Mich. Comp. Laws § 450.2307

Sec. 307. (1) A subscription for shares or membership made before or after a corporation is formed is not enforceable unless it is in writing and signed by the subscriber. (2) A subscription for shares of or membership in a corporation to be formed is irrevocable and the corporation may accept it for a period of 6 months, unless otherwise provided in the subscription agreement or unless all of the subscribers consent to its revocation. (3) A contract with a corporation to purchase its shares to be issued is a subscription agreement and not an executory contract to purchase shares, unless otherwise provided in the contract.